The present invention relates to a method for producing asymmetric radial polymers. More particularly, this invention relates to a method for producing asymmetric radial polymers having 6 to 8 vinyl aromatic hydrocarbon-conjugated diene copolymer arms and conjugated diene homopolymer arms and/or vinyl aromatic hydrocarbon homopolymer arms.
Heretofore, several methods have been proposed for preparing asymmetric radial polymers. As is well known in the prior art, radial polymers comprise three or more arms extending outwardly from a nucleus. The asymmetric radial polymers contain arms of at least two different polymers, which polymers may vary as to chemical composition, structure and/or molecular weight. A principle difference in the methods frequently used to prepare asymmetric radial polymers resides in the selection of a coupling agent which forms the nucleus of the radial polymer. Multifunctional coupling agents such as silicon tetrachloride have been used to form asymmetric radial polymers having three or four arms. Star-shaped radial polymers having many more arms have been formed using a poly alkenyl aromatic compound, such as divinyl benzene, as the coupling agent as described in Canadian Patent 716,645.
Before 1988, such asymmetric radial polymers were made by forming a blend of the different polymeric arms and then adding the coupling agent to couple the arms to the coupling agent. Those methods resulted in a product having, on average, the desired number of each kind of arm in the asymmetric polymer. The problem associated with producing asymmetric polymers in that manner is that the product obtained is actually a statistical distribution of all possible products. Unfortunately for enduses such as adhesives, this means that a certain percentage of the polymer produced would constitute a homopolymer polydiene radial polymer. If the overall polymer contains more than 5 weight percent of such homopolymer, it will be unsuitable for use in adhesive formulations because the polydiene homopolymer is mobile, bleeds to the surface of an adhesive formulation and thus adversely effects that adhesion of the formulation.
In 1988, one of the present applicants filed a patent application which resulted in the publication of European Patent Application 0,314,256. Therein is disclosed a two-step process for producing asymmetric radial polymers which avoids the problem of the production of the unwanted homopolydiene polymer. The process involves separately polymerizing the monomers to create separately the two different types of arms. Then one of the polymeric arms is coupled to the coupling agent and when that coupling reaction is complete, the second set of polymer arms is coupled to the coupling agent. This insures that there will be very little homopolydiene in the final polymer. The two-step process described above is very advantageous and produces polymers which have very good properties and are useful in adhesive compositions and for a wide variety of other uses. However, the two-step process does have the disadvantage that it is more expensive to carry out. It requires additional capital expenditure for a second reactor and additional time to produce the final polymer. Therefore, it would be very advantageous to have a method which was capable of producing in one step an asymmetric radial polymer which contains little or no homopolymer polydiene. The present invention provides such a process and produces such a polymer.